Previous studies have reported that clitic interpolation and clitic climbing change according to three stages in the history of European Portuguese. In this paper we develop the idea that these phenomena are deeply intertwined in the diachrony of Portuguese once they are related through the notion “clitic position” and reflect a general strategy for clitics to stay outside the focus domain of the clause. In order to provide a unified explanation for interpolation and climbing, we follow a quantitative methodology using the Tycho Brahe Corpus of Historical Portuguese. Moreover, developing some ideas about the Force-Fin system, in the Generative Grammar framework, we put forward a formal approach to the diachronic change having taken place between Old Portuguese and Classical Portuguese. Considering that clitic position reflects the category hosting the clitic, we argue that the availability of XP interpolation and of obligatory clitic climbing in Old Portuguese are different consequences of more basic structural principles, such as the V-to-Fin movement, which is dependent on the role of left-peripheral elements in the Old Portuguese grammar. Therefore the clitic could occur either in Force or in Fin, in order to mark the relative presuppositional nature of clitics with respect to different left-peripheral elements. In Classical Portuguese, we defend that XP interpolation paved the way for Neg interpolation because of the loss of V-to-Fin movement in embedded contexts; by then clitic climbing started to reflect a number of criteria related to topicality. Finally, in Modern European Portuguese, the gradual loss of Neg interpolation and the marked status of clitic climbing can be seen as further consequences of the loss of V-to-Fin movement.